Process of manufacturing perchloric acid by anodic oxidation of chlorine



g- 1958 w. MULLER 2,846,383

PROCESS OF MANUFACTURING PERCHLORIC ACID BY ANODIO oxnmron OF CHLORNE Filed July 2, 1957 CHLORINE H2O M PRODUCTION INVENTOR. WILHELM \MULLER BY HOMM ATTORNEYS United States Patent PROCESS OF MANUFACTURING PERCHLORIC ACID BY AN ODIC OXIDATION OF CHLORINE Wilhelm Miiller, Darmstadt, Germany, assignor to E. Merck Aktiengesellschaft Darmstadt, Darmstadt, Germany, a German corporation Application July 2, 1957, Serial No. 669,547

Claims priority, application Germany July 5, 1956 6 Claims. (Cl. 204-82) The method of producing perchloric acid as heretofore known is very involved and cumbersome in that; first, sodium chlorate is produced from common salt by electrolysis; and then the sodium chlorate, after isolation in concentrated solution is oxidized on platinum electrodes. By reacting the sodium perchlorate with concentrated hydrochloric acid, the perchlorate is converted into sodium chloride and free perchloric acid. The technical perchloric acid thus obtained contains, after separation of the sodium chloride, still small residues of sodium perchlorate in addition to other impurities contained in the starting material. The perchloric acid can be liberated from the residual sodium perchlorate only by distillation.

U. S. Patent 1,271,633 describes a method of producing perchloric acid by electrolysis from strongly diluted hydrochloric acid to 1 N). The resulting perchloric acid is very dilute (about 30 grams per liter) (compare the table on page 2 of the American patent) and the current consumption required is very high. According to the examples of the American patent the consumption of current in the best case is 37.8 kwh.;v and the current used is direct current which corresponds to a consumption of 3 phase current (at 80% efiiciency of the rectifier) of 47.3 kwh. per kilogram of 100% perchloric acid. The reason for the high current consumption is the low conductivity of the dilute acid used which causes a comparatively high cell potential. As a result, that method is not suitable for the commercial production of perchloric acid and the perchlorate and has not been so used (see the Chemical Age of November 13, 1943).

It has now been found that a highly economical method is achieved if one does not start with hydrochloric acid, but with elemental chlorine which is dissolved in a 10% to 60%, preferably 25 to 40%, aqueous solution of perchloric acid; and anodically oxidizes the elemental chlorine in such solution directly to perchloric acid.

The cathode material is preferably graphite, silver, platinum, copper or alloys of these three metals. 'The most suitable anode material is platinum and also the platinum metals and the alloys thereof.

The use of comparatively concentrated perchloric acid as an electrolyte permits the operation of an electrolytic cell with comparatively low potential whereby the resulting product is obtained in a high concentration.

In detail the method is carried out by cooling an aqueous 10% to 60% solution, preferably 25% to 40% solution of perchloric acid, outside of the electrolyte cell to a low temperature, preferably to 0 C., and saturating the solution with chlorine. This solution is passed through the anode zone of the cell wherein part of the chlorine is oxidized and returned by means of a pump into the chlorine saturating zone where it is recooled in addition to being again saturated with chlorine. The reaction:

requires water which is added to the circulating system continuously or discontinuously to maintain the concenice 2 tration of the circulating acid at the most favorable level. The produced acid is continuously withdrawn from the circulation system when leaving the cell. The most favorable current densities are between 20 and 30 amp./dm. and the most favorable potentials, 3.9 to 4.1 volts as appears from the following table:

, 7 Current Platinum Poten- Electro- Consumpinvesttial in chemical tion per ment per Current density, Volts Yield Kilogram 250 kiloamp./dm. H0104 grams of (Watts) H0104, 60% per day With this method there is practically no loss of anode metal as the evolved perchloric acid contains but traces of these metals. If, for instance platinum is used as anode metal, the platinum of the anode which is dissolved during the electrolysis deposits at the cathode and can be recovered therefrom.

The described method produces an acid with a considerable content of perchloric acid which contains only 2-3 grams of chlorine per liter. After evaporatingthe excess of water this content of chlorine disappears and there remains a 60% perchloric acid of the highest purity.

As the perchloric acid at the cathode does not undergo any change during theelectrolysis, the method permits, at least in principle, the use of a cell without separation of the anode space and the cathodespace for the anodic oxidation of the chlorine. However, it has been found that the electrochemical yield is higher when access of the cathodically produced hydrogen to the anode is prevented. The reason for this phenomenon is presumably that the evolved hydrogen reduces the solubility of the chlorine in the electrolyte. It has been found that the cathode space or zone may be adequately separated by a diaphragm or a thin tautly stretched web of synthetic material, preferably of polyvinyl chloride which, influences the potential of the cell only to an insignificant extent.

A more detailed description of the invention will become evident from the following examples when read in connection with the accompanying drawing:

Example 1 A perchloric acid containing an appropriate quantity of HClO is fed by means of a pump from a supply container 2 to a cooler 3. The acid is cooled in the cooler to the desired temperature and then conducted through the saturator 4 in which it is saturated with chlorine. It is then fed into the anode space or compartment of cell 5 in which part of the dissolved chlorine is oxidized to perchloric acid at the platinum anode. The acid is recycled into the container 2 from the anode space. Part of the anolyte passes into the cathode space or compartment through the polyvinyl chloride fabric separator to an extent such that the impoverishment of the catholyte occurring due to the migration of the C10 ions to the anode is just balanced. The flow into the cathode space is controlled by regulating the run off. After withdrawing off from the cathode run-off, the quantity of perchloric acid which has been produced in the cell, the remainder flows back into the container 2 and thus goes back into the circulating system.

The. water which is consumed during the electrolysis and water withdrawn due to the production of perchloric acid is also returned into the collecting container 2 and thence into the circulation system.

The completely pure acid which contains only minute quantities of dissolved chlorine (about 2 grams per liter) is concentrated to 60% whereby it is entirely stripped of free chlorine.

Numerical-example Cell potential-3.92 volt Strength of current1 250 ampere Density of current-20 amperes/dm.

Temperature of the solution as it enters the cell was I "emperature in the anode space 2 C.

The concentration of the perchloric acid was 40.5%.

Anode: platinum, Cathode: silver Period of time (operating)-24 hours Yieldl0.877 gr. HClO l%=l8.l28 gr. perchloric acid 60% Current efiiciency69.3

Example 2 The anodic oxidation of chlorine to perchloric acid was carried out, in accordance with the general procedure above described, by introducing a aqueous solution of perchloric acid containing dissolved elemental chlorine into the electrolytic cell. The oxidation was carried out at a current density of amperes/drn. The potential was 4.02 volts. The temperature of the solution of chlorine was 1 C.

Example 3 It will be understood that the foregoing description of the invention and the examples set forth are merely illustrative of the principles thereof. Accordingly the appended claims are to be construed as defining the invention within the full spirit and scope thereof.

, I claim:

1. Method of manufacturing perchloric acid which comprises anodically oxidizing, at a temperature of about 0 C., elemental chlorine dissolved in an aqueous solution of perchloric acid wherein the perchloric acid is present in an approximate concentration of 10 percent to 60 percent.

2. Method in accordance with claim 1 wherein the current density is about 20 to amp./dm.

3. Method in accordance with claim 1 wherein a diaphragm is interposed between the anode and the cathode.

4. Method in accordance with claim 1 wherein a synthetic fabric is interposed between the anode and the cathode.

5. Method in accordance with claim 1 wherein the anode is a member of the group consisting of platinum, platinum metals, and alloys thereof, and the cathode is a member of the group consisting of graphite, silver, copper, platinum and alloys of said metals.

6. Method of manufacturing perchloric acid which comprises anodically oxidizing, at a temperature of about 0 C., elemental chlorine dissolved in an aqueous solution of perchloric acid wherein the perchloric acid is present in an approximate concentration of 25 percent to 40 percent.

References Cited in the file of this patent UNITED STATES PATENTS 2,772,229 Karr Nov. 27, 1956 

1. METHOD OF MANUFACTURING PERCHLORIC ACID WHICH COMPRISES ANODICALLY OXIDIZING, AT A TEMPERATURE OF ABOUT 0*C., ELEMENTAL CHLORINE DISSOLVED IN AN AQUEOUS SOLUTION OF PERCHLORIC ACID WHEREIN THE PERCHLORIC ACID IS 